mysqli_change_user() is used to change the user of the specified
database connection as given by the link parameter and to set the
current database to that specified by the database parameter.

If desired, the NULL value may be passed in place of the database
parameter resulting in only changing the user and not selecting a database. To select
a database in this case use the mysqli_select_db() function.

In order to successfully change users a valid username and
password parameters must be provided and that user must have
sufficient permissions to access the desired database. If for any reason authorization
fails, the current user authentication will remain.

Note:
Using this command will always cause the current database connection to behave as if
was a completely new database connection, regardless of if the operation was completed
successfully. This reset includes performing a rollback on any active transactions,
closing all temporary tables, and unlocking all locked tables.